Poxviruses comprise highly dangerous emerging and re-emerging pathogens of humans and other vertebrates. They continue to have significant impact on public health despite eradication of smallpox, the deadliest disease in human history. Poxviruses are also being actively developed to treat various infectious diseases and multiple cancers. All poxvirus mRNAs transcribed after viral DNA replication have a poly(A) leader in their 5'-untranslated regions (5' UTRs). Though it is well established that the 5' UTR of an mRNA plays an important role in regulating eukaryotic mRNA translation, it is yet elusive whether the poly(A) leader facilitates viral mRNA translation during poxvirus infection. This lack of knowledge represents a major gap in understanding the fundamental mechanism of poxvirus gene expression. Using vaccinia virus, the prototypic poxvirus, the objective of this project is to determine the role of the poly(A) leader in translation of vaccinia virus post-replicative mRNAs and the mechanism by which the poly(A) leader functions. Our preliminary data support a hypothesis that the 5'-poly(A) leader confers a selective translational advantage specifically in vaccinia-infected cells that may involve both cap-independent and cap-dependent translation. The hypothesis will be tested through two aims. In Aim 1, the role of the poly(A) leader in translation of vaccinia post-replicative mRNAs will be determined. In aim 2, the roles of cap-dependent and cap-independent modes in translation of poly(A)-leader-containing mRNA will be examined. This project is expected to identify and establish a fundamental mechanism used by vaccinia virus to selectively translate its post-replicative mRNAs.